Conventional electrical transmission conductors, e.g., ACSR (Aluminum Conductor Steel Reinforced), are broadly used in electrical transmission and distribution networks. Newer conductors reinforced with composites of lower thermal expansion than steel are adopted in electrical transmission and distribution networks to increase capacity and efficiency while reducing cost and complying with electric grid requirements (e.g., reliability and safety), due to their superior high temperature low sag characteristics. These newer conductors use aluminum (fully annealed) or high temperature aluminum alloys, reinforced with strength members such as metal matrix or polymer matrix composites. ACSS Conductor (Aluminum Conductor Steel Supported) is another high temperature conductor, and it uses annealed aluminum for high temperature operation.
The thermal knee point is relevant in conductors made of differing materials (e.g., strength member vs. conductive member) and is defined as the temperature above which the conductive constituents in the conductor are no longer carrying tensile load or are in compression. The conductive constituents in these conductors, such as aluminum, aluminum alloys, copper or copper alloys are typically under tensile stress after conductor stringing, resulting in thermal knee point higher than the majority of operating temperatures. Until the conductor reaches above its thermal knee point, the conductor thermal expansion is substantially controlled by conductive material such as aluminum or copper with high thermal expansion coefficient, resulting in large sag, limiting the conductor's current carrying capacity, as shown in FIG. 1. This is especially significant for conductors in reconductoring applications or in long span applications where thermal sag often becomes the limiting factor for increasing current carrying capacity in electric transmission and distribution network.
Besides the constituent material's properties, conductor thermal knee point is also affected by the conductor's tension and its tension history.
Gap conductor is a special high temperature conductor with low thermal sag by suppressing conductor thermal knee point. This was accomplished by suppressing the thermal knee point in Gap conductor during special conductor installation procedure. Gap conductor is made with steel wires and high temperature aluminum alloys where a precisely controlled gap between the steel core (i.e., strength members) and the inner aluminum strand layer is maintained and filled with high temperature grease to facilitate relative motion between steel wires and the aluminum layers in conductor installation operation. Gap conductor must be installed by tensioning the steel wires (after stripping the aluminum layers to expose the steel wires) between transmission deadend towers. This tensioning process can be as long as 48 hours or more, and requires special device and extra labor time from linemen as the linemen have to revisit the towers for final deadending after the tensioning process. When properly installed, the conductor does exhibit low thermal sag as its thermal knee point is at or close to the installation temperature, and the conductor thermal sag is only controlled by the thermal expansion of steel wires (whose thermal expansion coefficient is about half of that of aluminum). However, Gap conductors are typically very expensive. It is difficult to install, requiring special training and tools and significantly more labor time in the field. Furthermore, since the conductor strength member is taking virtually all the load and it retracts inside the Gap conductor's aluminum layers if the conductor breaks, it is impossible to repair gap conductor in the field. The entire conductor segment from deadend to deadend must be replaced and installed, resulting in costly delays in restoring electrical transmission. The grease inside the gap conductor has being reported to leak out through the aluminum strands over time, staining objects under the power lines as well as corona noise due to water beading on conductor surface as a result of the hydrophobic greasy surface. The grease in Gap conductor is also for protecting the steel wires from corrosion, and removal of the grease will result in compromised corrosion resistance of gap conductors.
Another approach in getting low conductor thermal knee point is discussed in Chinese patent CN102103896A1, which mentioned a process of stranding annealed aluminum on the periphery of the steel core wires, while the bearing steel core wires are subjected to pre-stress treatment. The resulting conductor is claimed to be capable of continuous operation at temperatures up to 150° C. The product, made from this patent, was introduced to a major Chinese transmission project in 2013 for commercialization, where the conductor failed in field installation due to extensive birdcaging and uneven sag, and had to be replaced with conventional conductors and further application was prohibited by State Grid Corp of China. The patent did not discuss thermal knee point, or disclose the extent of pre-stress level, the stress level in aluminum strands, or the exact process and setup for pre-stressing core wires. The annealed aluminum strand, which readily deforms, likely bulged outwards when tensions in the steel core wires were released from the high level during pre-stress. When the conductor is wrapped in the take-up reel as typically done during conductor stranding manufacturing, the overlaying of these pre-stressed multi-strand conductors likely caused irreversible deformation of the annealed substantially loose/open aluminum strands in all the under layers of conductors. These permanent deformation of aluminum strands will cause not only conductor birdcaging, but also localized deformed aluminum strands to break and causing hot spots and conductor failure during energized conductor operation. Similar approach for thermo-resistant aluminum alloy conductor were also attempted in 20022, by JPS without much better commercial success. The severely loose aluminum alloys strands posed same challenges. The core in the conductor might be protected with a thin aluminum cladding in JPS approach for high temperature operation, however, the aluminum cladding on the core is also subjected to extreme tension as high as 190 MPa during the pre-stretching process of the core while aluminum strands are stranded, making it vulnerable to vibration fatigue. The thin cladding is unable to sustain the tensioned core and minimize its shrinking inside the conductor that the ends of the conductor must be fixed before the tension in the core is released, forcing all the aluminum strands to be very loose. The loose aluminum strands and the need to fix the conductor ends make it difficult to handle the conductors in both manufacturing and field stringing.
High temperature conductors, such as INVAR3 and ACCR4 conductors, with their constituent materials capable of sustained operation at high temperatures, use Al—Zr high temperature alloys. These conductors typically have high thermal knee points, often approaching or above 100° C., well above their everyday operating conditions (see table 1). Pre-Tensioning of conductors in the field is rarely attempted.
Pre-tensioning of ACSS conductors are occasionally done. This is accomplished when the ACSS conductors are already in and between towers, and a significant level of tension stress (e.g., a load equivalent to 40% conductor rated tensile strength) is applied to the conductor for hours before deadending. Pre-tensioning of ACSS does reduce thermal knee point and improve thermal sag, however, the high stress required in ACSS in tensioning increases risk to the safe operation of the transmission towers, especially for older transmission towers in reconductoring application projects.
There have been greater acceptance for conductors with strength member(s) made of fiber reinforced polymeric matrix composites and stranded with annealed aluminum, such as ACCC by CTC Global5, C7 by South wire, Low Sag from Nexans6, and other similar types during the past decade. These conductors are typically supported by carbon fiber reinforced composite as strength member(s), and an insulating layer(s) on top of carbon composite between carbon core and aluminum to prevent galvanic corrosion. The carbon composite core has one of the lowest thermal expansion coefficients, and these conductors are very low in thermal sag above thermal knee point, and can be operated to temperatures as high as 200° C., delivering significantly higher ampacity than ACSR conductors (when needed such as N−1 emergency situations). These conductors are strong and light weight, and the composite strength member(s) are resistant to corrosion associated with steel types of strength members.
These composite core conductors, however, typically have thermal knee points of 70° C. or higher. Below this temperature, the conductor thermal elongation is dominated by the aluminum strands, exhibiting substantial thermal sag. Virtually all these conductors are used in reconductoring for capacity expansion to leverage existing infrastructure and the existing right of way. It is uncommon for these conductors to be pre-stressed on existing towers as the older towers may not be capable of high level pre-tension required to substantially suppress conductor thermal knee point. These composite core(s) are vulnerable to fiber buckling failure from excessive axial compressive stress during installation, such as the case in sharp angle situations associated with mishandling. Conductors with smaller cores, with better bend flexibility, are ironically more vulnerable as these conductors do not require much bend stress to fail when subjected to sharp angle (with the aluminum strands in the stranded conductors, sliding to accommodate bending of the strength member), especially when tension on the composite strength member is absent. If the core suffers only partial damage, the conductor failure could be delayed by months or years after the initial damage, posing serious threat to the safety and reliability of electricity transmission network.7 A composite core conductor, that is robust against mishandling and whose strength member is under substantial pre-existing tension while the conductive constituents are substantially tension free, would be very desirable for safe handling and installation and necessary for the safety and reliability of the electric transmission and distribution network.
While the annealed aluminum in these conductors offers maximum electrical conductivity, they readily deform under tensile stress. These conductors rely on the core for mechanical load, typically requiring special hardware fittings to secure the core(s). Hardware costs for projects using such conductors sometimes are as high as 50% of total project cost, which is unacceptable, especially for cost sensitive applications such as lower voltage electrical distribution network. Expensive special fittings such as collet housing approach from CTC Global or aluminum sleeve approach inside the compression fitting from AFL must be used with conductors with composite strength members. Furthermore, these conductors must follow precisely prescribed stringing temperature and time duration, especially in bundled configurations during stringing, making the installation process prohibitively expensive. If the tension and time history of the phase conductors are different, there could be different thermal knee points for each conductor and differential sagging among the bundled phase conductors after installation, causing flashing or even short circuits with changing conductor temperatures. For example, in a 220 kv ACCC recondcutoring project in china in 20118, the field engineer reported that the sags of phase conductors (ACCC Drake) exhibited large variation despite the same stringing tension of 18 KN. One conductor was clipped in on Mar. 30, 2011, and the conductor sag had significantly increased by 0.69 m when observed on April 2nd and by 0.77 meters on Apr. 3, 2011. Two other phase conductors in the same circuit and at the same location were clipped a day later on Mar. 31, 2011 under identical stringing tension of 18 KN, and the sags of each conductor were observed to increase by 0.9 m on April 2nd and 1.175 m on April 3rd for one conductor, and by 0.78 m on April 2nd and 0.86 m on April 3rd for the other conductor. Such changes in conductor sag are not only substantial but also seemed random and unpredictable, a significant issue for field engineers and the electric utility. If these conductors are already at low thermal knee point (and preferably without the need to pre-tensioning in the old towers in such a reconductoring project), one could install these conductors at ease to get target sag clearance during and after stringing without the sensitivity to installation practice (e.g., variability in the stringing time, stringing temperature, stringing tension among phase conductors).
Another challenge for conductors with carbon fiber polymeric composite core and annealed aluminum is their high sag in heavy ice environments. To avoid excessive stringing tension load onto the towers while maintaining sag clearance, engineers sometimes adjust the conductor to further improve sag after the conductor was subjected to ice loads for the first time where the conductor tension drops after ice load. This requires extra time and expensive effort from linemen. If these conductors are already at low thermal knee point without high degree of pre-tension treatment in tower, one could install these conductors at higher clearance without increased tension to electrical tower, thus better able to handle sag from heavy ice loads. This procedure will be unnecessary if a pre-tension treated conductor is used.
In electric distribution network, where it operates at lower voltage, conductors are subjected to higher current density due to cost constraints. With increasing difficulty in securing right of way to build new electric transmission and distribution network, it is highly desirable for high temperature conductors to be deployed for distribution that can substantially increase capacity when needed in emergency, while delivering good energy efficiency. These are typically smaller conductors, and it is important to have a conductor system solution that is cost effective (in conductor, in fittings and installation) as well as easy to install, maintain and repair.
Accordingly, there remains a need for knee point suppressed conductor capable of high temperature operation without the need for conductor pre-stressing at the electric towers that may compromise the tower safety. Furthermore, it is desirable to have a conductor solution using composite strength member that is cost effective, easy to work with (installation consistency and free of birdcage, robust against mishandling in the field, easy to repair and maintain, better energy efficiency, ultra-low sag, and compatibility with existing fitting). The present invention solves these issues by providing a complete conductor system solution that is cost effective (conductor, installation, repair and hardware), high capacity and energy efficient, low sag under high temperature and heavy ice, and virtually no sag change with temperature variations by ensuring the strength member(s) in the conductor is under pre-stressed condition while substantial amount of the conductive media is under no tension or under compression without damaging the conductor integrity (e.g., birdcaging) prior to conductor installation onto the towers.